I was under the impression that the motors Williams used were rated for 12-30 volts. Right-of-Way made a 400W transformer that went up to 24 volts, and Lionel's prewar Z transformer did, as well. This kind of voltage and current are required to make these locos run at prototypical passenger speeds (IMO that only makes sense if you have a very long layout!)
Yes, things get noisy at high RPMs and the brass shell amplifies that sound. There's a reason they don't make tubas out of die-cast metal! I discovered that a piece of DynaXorb in the boiler shell quiets things down a lot.
Mike Wolf and Andy Edelman had a lot of experience selling Williams. My theory is when MTH started making "scale" trains, they purposely chose a tall gear ratio to keep the RPMs down, because they didn't want to field complaints about NVH (noise, vibration, and harshness.) Unfortunately, low-speed performance in the Proto-1 era left a LOT to be desired.